I woke up this morning to a screeching alarm clock, clicked on my bedroom light, and set to work on the my usual morning routine of coffee and radio. My electric kettle sat hissing and crackling in its struggle to bring a stiff cup’s worth of coffee water to a boil. Following the kettle’s cord back to the kitchen wall, I imagined hulking electrical apparatus as it stretched away from my kitchen: my kettle’s cord connected to the outlet, my household wiring, a Pacific Gas and Electric Smart Meter, neighborhood distribution lines knotted up in trees, substations, high voltage distribution, and, finally, generation plants. These plants, the mitochondria of the modern societal organism, are scattered just-out-of-sight at the outskirts of nearly every population center in America. What do they eat for fuel? How do they work? And why is their favorite food, coal, a dwindling industry and hot button issue in American politics?

America has a huge appetite for electricity. In 2015, we collectively consumed 4 trillion kilowatt hours of electrical energy. If you don’t have an intuitive grasp of how much a kilowatt hours is, think of it as the amount of energy consumed by a microwave oven operating at full power for one hour. That’s a lot of Hot Pockets. In November 2016, the average price of electricity was $0.10 per kilowatt hour, so our national power bill comes out to about $30 billion per month. Thank God for roommates.

Where does all of that electricity come from? Just over 90% of all American electricity is generated by thermal (heat) power plants. Thermal power plants all operate in the same way: they use some fuel to get a fluid hot, then use that fluid to turn a turbine and a generator. Wait, what? Think of it like this: replace the word “fuel” with “coal” and replace the word “fluid” with “steam.” Coal-fired thermal power plants use coal to heat up water into steam, then we that hot high-pressure steam to turn a turbine which in turn (har!) rotates a generator.

Amazingly, though it may seem like a relic from the 19th century, almost all electricity in the United States is still generated by using fire of some sort to turn water into steam. Then we use that steam (and hot exhaust sometimes, too) to do useful work. Those scary photos you always see at the top of articles about pollution and climate change showing huge billowing clouds rising out of smokestacks? Thermal power plants generate lots of exhaust, but almost all of that exhaust is made up of some combination of water vapor and carbon dioxide. Power plant stock photographers do most of their work on cold winter days when the water vapor most dramatically condenses into clouds. In America, the old days where smokestacks earned their namesake are, mercifully, over.

Figure: a conceptual model of all thermal power plants

This is where things get interesting. The modern debate over fueling our massive electricity infrastructure essentially boils down to one simple question: who can turn heat into electricity in most economically? The most important heat sources for generating electricity in the United States are, in order of decreasing importance: coal fire, natural gas fire, nuclear fission, biomass, and petroleum liquids. In the conceptual model of a thermal power plant, these heat sources are the “fuels” for the heat generation process. Coal, natural gas, biomass, and petroleum liquids generate carbon dioxide and water as primary waste products, while fission generates nuclear waste materials.

Figure: breakdown of the most important thermal energy sources for electricity production in the USA (EIA, 2015)

For the remainder of this article, let’s focus on the big hitters: coal and natural gas. You might think that, if the goal is to turn heat into electricity as affordably as possible, we should buy the energy source that is the cheapest per heat generated. Unfortunately, you would be wrong. The energy industry loves antiquated units, but bear with me as I give you the rundown on how much coal and natural gas cost in the United States today. According to the most recent Energy Information Administration data at the time of writing, the national price paid for coal ranged between $11.80 and $50.05 per short ton (why such a spread? more on that later), and the average price for natural gas for electricity production purposes was $3.14 per thousand cubic feet. So, how many short tons of coal are there in a cubic foot of natural gas? Well, that question is a bit like asking how many miles there are in a gallon. Let’s get both energy sources into non-antiquated units for a more meaningful look. One short ton of coal contains about 21 billion joules of heat energy, and, similarly one thousand cubic feet of natural gas contains 1 billion joules. Divide the cost by the energy content, and what we find is that US coal costs $0.56-$2.38/gigajoule and natural gas comes in at $3.14/gigajoule. Coal seems like a clear win at just 18% - 76% the cost of natural gas per unit of thermal energy, so why is coal power and extraction dwindling while natural gas extraction and power production boom?

When we think of the real cost of generating electricity, we have to remember that the important denominator is electrical energy, not heat energy, and that’s where natural gas has a secret weapon. The average American coal-fired power plant has a thermal efficiency of 33%. In other words, only 33% of the joules that are generated as heat get turned into electrical energy joules; for every 100 coal fire joules in, we get 33 electricity joules out. By contrast, today’s modern Mitsubishi, Siemens, and GE natural gas power plants are achieving efficiencies in the 57% - 59% range. How is it possible that one fossil fuel source can turn much more of its thermal energy into electrical energy than another?

Coal-fired power plants operate on the steam cycle explained earlier, which is technically termed the Rankine Cycle. In this cycle, coal fire heats water, turns it into steam, steam turns a turbine, and the turbine turns a generator. By contrast, today’s most-efficient natural gas power plants operate on a combined cycle. First, the natural gas is injected directly into a gas turbine in a cycle called the Brayton Cycle. This gas turbine, which is for all intents a purposes a jet engine whose job is to create rotational power instead of thrust power, burns the natural gas and rotates its blades to turn a first-stage generator. Then, the exhaust exits the gas turbine (the “not-so-hot fluid” from the earlier schematic), and is used to power a secondary Rankine Cycle. The hot exhaust boils water, makes steam, and powers a Rankine Cycle to turn a second-stage generator. In this way, a natural gas combined-cycle power plant achieves significantly higher thermal efficiencies than coal. How come coal cannot be burned in a combined-cycle power plant at higher efficiencies? We don’t have the technology to burn coal dust directly inside a Brayton Cycle turbine without ruining the turbine. Wasn’t the title of this article “Where Have All the Coal Boys Gone?” What’s with all this thermodynamics drivel? Trust me, all of this matters.

Figure: a schematic of a combined-cycle power plant courtesy of Mitsubishi Hitachi Power Systems

Because natural gas combined-cycle power plants can turn heat into electricity more efficiently than coal-fired plants, they can save money on fuel costs. While US coal costs $0.56 to $2.38 per gigajoule and natural gas comes in at $3.14 per gigajoule, the efficiency-adjusted fuel cost for generating coal-fired electricity comes out to $0.006 to $0.026 per kilowatt hour for coal and $0.019 per kilowatt hour for natural gas.

Still wondering where the coal boys have gone? Stay tuned for my next blog, where I identify what really killed the coal industry (hint, it’s not regulation).

About the Author

DANIEL WILSON, PH.D

Danny is a gritty get-it-done engineer with penchant for design, hardware, manufacturing, and team leadership. Danny is especially interested in hardware and software tools for reducing impacts of the energy sector. Danny is the founder for Geocene, a sensing and analytics company. Danny holds a Ph.D. in mechanical engineering from U.C. Berkeley, and is a former Fulbright Fellow and National Science Foundation Fellow. His past experience includes developing sensors and web applications for measuring energy use in context ranging from skyscrapers in Chicago to rural huts in Ethiopia. Daniel has also worked in the aerospace industry designing landing gear for Boeing and Airbus, and he has served as Chief Technology Officer of Persistent Efficiency, an internet of things company focused on simpler monitoring of electricity usage. He is currently a postdoctoral fellow at Lawrence Berkeley National Laboratory focusing on scaling up several hardware and software products for monitoring technology adoption.